The term “valve metal” refers to metals such as vanadium, tantalum and niobium that are often utilized in valves, such as the intake/exhaust valves in engines. A commercially valuable form of a valve metal is a valve metal oxide such as a tantalum pentoxide, (Ta2O5) or a niobium pentoxide (Nb2O5) which are produced from mineral ores.
Minerals of concentrates containing tantalum and niobium are conventionally extracted with hydrofluoric acid (HF) or mixtures of hydrofluoric acid and sulfuric acid (HF/H2SO4). The tantalum and niobium heptafluoro complexes formed are typically separated by solvent extraction.
In a conventional process for producing tantalum pentoxide (Ta2O5), the tantalum fraction from the ore and solvent extraction is stripped into the aqueous phase, and tantalum pentoxide is precipitated using ammonia and recovered by filtration. Niobium pentoxide may be produced in a similar fashion.
Typical conventional processes for producing niobium pentoxides/hydrates and tantalum pentoxides/hydrates are batch processes Disadvantages inherent with batch processes include the need to clean and reload processing vessels, that batch size is limited to the size of the processing equipment, and that the production of large quantities of material requires multiple batch runs.
In addition, in conventional processes for producing tantalum pentoxides/hydrates and niobium pentoxides/hydrates it is difficult to control the particle size, and the particle size distribution of the pentoxides and hydrates produced.
For many applications, it is desirable to have a tantalum pentoxide, or niobium pentoxide, product with a consistent particle size (i.e. a narrow particle size distribution). In addition, for many applications it is desirable to have a tantalum pentoxide, or niobium pentoxide, having large dense spherical particles (a particle size greater than or equal to 5 micrometers (μm)) and a bulk density of 2.0 g/cc or greater for Ta2O5 and 1.2 g/cc or greater for Nb2O5. For other applications low bulk density fine particle sizes are preferred (a particle size less than or equal to 5 μm) and a bulk density of less than 2.0 for Ta2O5 and less than 1.2 for Nb2O5. The present invention advantageously allows the production of tantalum pentoxide, or niobium pentoxide, products with narrow particle size distributions within a desired particle size range.